Optical discs, such as compact discs and minidiscs used for music, DVDs used for movies or the like, and MO discs and CDs-R/W used for, for example, data recording in computers, are presently available.
In optical disc apparatuses that access such an optical disc, that is, perform recording and/or reading, the size of beam spots formed on an information recording surface of the optical disc has been reduced for a higher recording density by shortening the wavelength of a light beam to be applied to the optical disc and increasing the numerical aperture of an optical system.
However, when an optical disc is accessed with an objective lens having a high numerical aperture, spherical aberration is caused by variations in thickness of a light-transmitting layer in the optical disc.
While, in general, it has been considered to increase the numerical aperture of the objective lens by adopting a so-called duplet lens, spherical aberration is also caused by variations in distance between the lenses in this case.
In a case in which the recording capacity is increased by adopting a multilayer information recording surface in the optical disc, spherical aberration is also caused by such a multilayer structure.
In order to correct such spherical aberration, an optical unit has been proposed in which a spherical-aberration-correcting element is provided between an objective lens and a laser source and in which the aberration-correcting element is combined with the objective lens (Japanese Unexamined Patent Application Publication No. 2002-117568 (FIG. 1)).
The optical unit includes an actuator for moving the optical unit in the tracking direction and the focusing direction. An elastic member for supporting the optical unit is made of a conductor, and the conductor is used as a wire.
A signal for driving the aberration-correcting element is superimposed on a signal for driving the actuator and is supplied to the wire. In the optical unit, the driving signal for the actuator and the driving signal for the aberration-correcting element are separated by a signal separating means.
In such an optical unit in which the aberration-correcting element and the objective lens are combined, however, noise is readily produced in the driving signals, and this decreases the S/N ratio and deteriorates the characteristics. Moreover, since means for superimposing and separating the driving signals is necessary, the configuration is complicated, and the production cost and parts cost are increased.
In a case in which an independent wire member only for the aberration-correcting element is provided without superimposing and separating the driving signals, the elastic member (suspension) for supporting the optical unit may be influenced by the weight and tension of the wire member for the aberration-correcting element, and the actuator may be unable to maintain desired dynamic characteristics.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide an actuator that reliably ensures desired characteristics in an optical pickup and that advantageously simplifies the configuration.